1. Field of the Invention
This invention relates to gas turbine engines and more specifically to the blades of the compressor rotor assembly.
2. Description of the Prior Art
Scientists and engineers within the turbine engine field have long recognized that vibratory damage adversely limits the life of many turbine machines. They have also recognized that the blades of the rotor assembly are among the most susceptible of compressor components to vibratory damage. The blades are of necessity designed for low weight in order to minimize the centrifugally generated loads on the rotor. Lightweight blades, however, are not always compatible with the durability requirements of the engine and may severely limit the operating life of the engine where the natural frequency of the blade system in cycles per second falls within the operating range of the engine as expressed in revolutions per second, or a multiple thereof.
Where the operating speed of the engine in revolutions per second, or multiple thereof, is equal to the natural frequency of the blade system each of the individual blades begins to resonate. At resonance a vibratory deflection of large amplitude is induced by relatively small amplitude stimuli as the stimuli act in reinforcing concert with the periodic deflections of the blade. The large amplitude deflections produce severe mechanical stresses in the blade material and ultimately cause fatigue failure of the blade.
Blade deflecting stimuli are produced by nonuniform pressure patterns causing each blade to be cycled from low loading conditions to higher loading conditions. The variation in loading characteristics induces blade deflection and imposes a strain on the blade material. At resonance the natural frequency of each installed blade is coincident with the frequency of the stimulus. The deflection amplitudes become reinforcing and vibratory damage as discussed above results.
Struts or other protuberances within the engine flow path precipitate nonuniform pressure patterns. In the case of a single protuberance, a vibratory excitation, which is referred to as the 1E, one excitation per revolution of the rotor, excitation is established. The frequency of the 1E excitation in a gas turbine engine is normally below the fundamental natural bending frequency of the blade system and is rarely of concern to engine designers. The 2E excitation occurs as two pulses are generated for each revolution of the rotor. The 2E excitation often coincides with the fundamental natural bending frequency of the desired blade system. The 2E excitation is a particularly severe problem for front end blades, such as fan blades, as the traditionally preferred geometric shapes and contours have inherent natural frequencies which approximate the 2E excitation frequency within the engine operating range. Vibratory damage at the 2E frequency must be avoided by reducing the amplitude of the stimuli, by altering the natural frequency of the blade system, or by removing energy from the blade system through mechanical damping of the blades.
One blade system employing frictional damping apparatus for removing a portion of the vibratory energy is U.S. Pat. No. 3,314,652 to Geberth et al. In Geberth et al mechanical links, which join the tips of the rotor blades, frictionally damp the blades in response to centrifugally generated forces. The damping removes energy from the blade system to mitigate the adverse effects of torsional and bending vibration. Frictional damping has a limited potential for the control of vibratory damage in gas turbine engines. The amount of energy developed within the blade systems, and particularly within the fan blade systems, of modern engines exceeds the amount of energy that can be effectively dissipated by frictional damping apparatus without causing substantial wear on the friction surfaces of the apparatus. It is, therefore, that engine designers are exerting every possible effort to avoid blade systems which rely on frictional damping for long term system protection.
Alteration of the blade system natural frequency is considered to be an alternative to frictional damping. An understanding of the concepts involved is gained by focusing on the effects that stiffness and mass have upon the natural frequency of blade systems. The natural frequency of a cantilevered blade system is proportional to the square root of the stiffness divided by the mass. EQU f.about..sqroot.(k/M)
where
f = natural frequency of the blade system, PA1 k = stiffness, and PA1 M = mass.
Increasing the mass or decreasing the stiffness lowers the natural frequency. Decreasing the mass or increasing the stiffness raises the natural frequency. Such variations are employable to drive the natural frequency out of coincidence with 2E frequency within the engine operating range.
One current practice for avoiding destructive vibrations in gas turbine engines is to raise the natural frequency of the blade systems above the level of the 2E stimulus at maximum rotor speed. This is accomplished by adding part span or tip shrouds, increasing the stiffness of the system, or decreasing the mass of the system. Decreasing the mass of the system rarely results in a significant rise in the natural frequency as the removal of mass generally produces a corresponding decrease in stiffness. An increased mass, however, if judiciously distributed for increased stiffness will raise the natural frequency. It is this technique and the addition of part span or tip shrouds which are most commonly utilized in engines today.
U.S. Pat. No. 3,044,746 to H. Stargardter entitled "Fluid-Flow Machinery Blading" teaches the optimized use of blade mass to increase bending stiffness while maintaining adequate torsional stiffness to resist self-excited vibration. Such teachings of optimized mass distribution notwithstanding, any added system weight such as that added to stiffen blades raises the centrifugal loads which must be carried by the rotor.
Substantial efforts are continuing within the gas turbine industry to develop lightweight blade systems which avoid destructive resonant frequencies while maintaining high torsional stiffness.